1. Field of the Invention
The present invention relates to a fiber optic Sagnac interferometer for measuring rotation rates. More particularly, this invention pertains to such an interferometer in which bias errors due to electromagnetic cross-couplings are substantially reduced.
2. Description of the Prior Art
Rotational movements produce phase shifts between the two oppositely circulating light beams of a Sagnac interferometer. As a result a rotation-dependent interference image is formed by superposition of the two beams.
The interferometer transfer function is defined as: EQU I=I.sub.0 .multidot.1/2cos (.DELTA..phi.)
Where I.sub.0 is the light intensity in the absence of a phase shift (please refer to FIG. 1). The phase shift (Sagnac phase) is related to the speed of rotation .OMEGA. as follows: ##EQU1## Where: L=length of optical path;
D=diameter of a circular optical path; PA0 .lambda.=wavelength of the light; and PA0 c=velocity of light.
The above-described transfer function possesses two considerable disadvantages when applied to a (rotational) sensor. First, the function is insensitive to small input values at the maximum of the cosine curve. Secondly, the sign of the phase shift cannot be determined from the intensity signal.
To remedy these problems, it is customary to shift the working point on the characteristic into the region of greater sensitivity through phase modulation. Two types of modulation frequently employed are sinusoidal as shown in FIG. 2a and square-wave as shown in FIG. 2b. The action of such a signal upon of the optical phase modulator produces a sensitive transfer function that is capable of recognizing sign.
Synchronous signal demodulation (shown in FIG. 2) provides the phase shift and, thus, the speed of rotation.
FIG. 3 is a block diagram of a Sagnac interferometer (rotation rate sensor) in accordance with the described prior art. The device includes a closed fiber loop, a light source L such as a laser whose parallel light beams are polarized by a polarizer P and split into light beams by a beamsplitter ST2 and injected in opposite directions into an interferometer fiber coil FS. The coil FS preferably consists of optical monomode fiber. The beam splitter ST2 also serves as a mixer for recombining the light beams after passing through the fiber coil FS. After passing through the polarizer P, the interference signal of the two superposed light beams passes via a second beamsplitter ST1 and output branch OUT to a photodetector PD that scans the intensity of the interference image. If .DELTA..phi..sub.0 were to designate the phase difference between the two counterpropagating light beams in the closed fiber coil FS, then, in the absence nonreciprocal disturbance, .DELTA..phi..sub.0 =0.
The electrical signal VD from the photodetector PD is accentuated by an impedance converter and amplifier A.sub.0 whose output signal VD' feeds a synchronous demodulator SYNCD that is synchronized with the modulation frequency f.sub.0. The demodulated output signal passes through an amplifier A as signal VA to an output interface S whose output signal is proportional to the rotation rate and contains the sign information that identifies the direction of rotation.
The principle of operation of the readout process for fiber-optic rotation rate sensors, as described with reference to FIG. 3, leads, in the operation of inertial devices for measuring rotation rate, to considerable difficulties (described below).
The voltage level of the modulation signal, VC or VC', to the phase modulator PM is on the order of a few volts in magnitude. However, the voltage level of the detector signal VD is a few nanovolts, corresponding to an apparatus rotation speed of 1.degree./h. Undesired stray effects of VC and VC' on VD lead to falsification of the measurement signal. The signal processing output includes apparent rotation rates. The null or working point of the measuring arrangement is altered by inherent disturbances, such stray effects indicated in FIG. 3 by broken lines and the coupling factor K.
It would be clearly desirable to eliminate (or at least reduce) electromagnetic strays (disturbances having a coupling factor K; refer to FIG. 3) by shielding measures and incorporation of filters into signal and voltage feed lines. This is indicated in FIG. 3 by shielding of the connecting line from the driver amplifier AP to the phase modulator PM. In the known interferometer of FIG. 3, this leads to particular difficulties. The signal VC (or VC') contains the modulation frequency f.sub.0 generated in an oscillator OSC. The photodetector signal VD, detected in the synchronous demodulator SYNCD, contains rotation rate information at the same frequency and phase relationship. The circuit assemblies that generate the modulation of frequency f.sub.0, and the portion of the circuit for conducting the signal sensitive to rotation rate (possessing the same frequency) are closely spaced and must, as a rule, be fed from a common power supply device. Thus, it is clear that a danger of stray electromagnetic energy of frequency f.sub.0 entering into the sensitive signal path (signal VD) exists. The addition of filters for blocking f.sub.0 to the signal lines is not possible since the desired signal information is present at precisely this frequency. Thus, undesired stray signal energy can be reduced to a limited extent only by, for example, shielding the amplifier A.sub.0 and the synchronous demodulator SYNCD against the remainder of the circuit and filtering the power supply. Despite such measures, the above-mentioned level conditions reveal that some coupling between the signals VC, VC' and VD is unavoidable regardless of all possible screening measures.